The raw leadframe material for leadless plastic chip carrier packages is generally presented in a strip format to facilitate the IC assembly process. Each strip can contain hundreds or thousands of individual Quad Flat Package No Leads (hereinafter, “QFN”) units, depending on the QFN body size. While in strip format, all of the leads (or terminals) of each individual unit, as well as all the units in that strip, are connected together by a segment of the leadframe metal called the dambar. This connection has been necessary to provide the mechanical rigidity required during fabrication and assembly, as well as to provide electrical connection during external lead finish using electrodeposition methods.
In order to perform final electrical testing of the QFN units, it is necessary to electrically isolate the leads. Known methods of isolation include:
Mechanical removal of the dambar using a circular saw that cuts approximately halfway into the plastic package, leaving the QFN units in the strip format for subsequent processing. This is a slow operation, and creates a structurally weak array of IC components in the finished leadframe strip, which may lead to mechanical failures during testing.
Mounting of a fully assembled and molded leadframe strip on an adhesive tape surface, followed by mechanical isolation (or singulation) of the individual units by saw-cutting through the full leadframe strip thickness, and leaving the singulated units on the sticky tape/metal ring assembly used during singulation for subsequent processing. The resultant structure is bulky and the tape tends to expand, thereby affecting the positional accuracy of each individual unit.
Mechanical isolation (or singulation) of the individual units in the strip and transferring the units to various shipping containers, such as tubes, reels of cavity tape, trays or canisters. The units are subsequently tested by taking them out of these containers and transporting them individually to the test equipment (one unit at a time) using various handling methods such as gravity feed, pick and place or vibratory bowl feeders. This is often a complex, expensive and time-consuming process.
In another form of the QFN, individual units are molded on a leadframe strip, leaving leadframe metal segments exposed between individual units in the strip. Units are then electrically isolated by severing the exposed metal segments that electrically connect the leads and units in the strip. This is generally accomplished by use of a precision mechanical punch tool. This requires unique punch tooling for each device body size and lead spacing, increasing process cost. The punch operation also increases risk of delamination between metal leadframe segments and the molding compound.